Fatty acid synthase (FASN) is a multi-enzyme protein complex that catalyzes the synthesis of fatty acids involved in energy production and storage, cellular structure and formation of intermediates in the biosynthesis of hormones and other biologically significant molecules (Nature Reviews Cancer, 2007, 7, 763-777). FASN is composed of two identical 272 kDa multifunctional polypeptides. As its main function, it catalyzes the synthesis of palmitate from acetyl-CoA and malonyl-CoA, in the presence of nicotinamide adenine dinucleotide phosphate (NADPH). In normal human tissues (with the exception of liver and adipose tissue), fatty acids are preferentially acquired from the diet, and expression of FASN levels are low. In contrast, FASN expression and activity is highly elevated in several pathological states including cancer, inflammatory and metabolic diseases. In particular, evidence shows that increased endogenous fatty acid synthesis is critical for tumorigenesis.
Cancer is a disease of accelerated cell growth and proliferation. Cancer cells adapt metabolically to increase levels of lipids to support their anabolic requirements. Increased synthesis of fatty acids represents a fundamental metabolic adaptation of cancer cells and is facilitated by high levels of FASN expression. Increased expression of FASN is an early event in tumorigenesis and is found in numerous tumor types, often correlating with a poor prognosis (Nature Reviews Cancer, 2007, 7, 763-777). FASN gene amplification and protein overexpression was observed in human breast, ovarian, prostate, colon, lung, bladder, stomach and kidney cancers suggesting FASN as a potential drug target and marker of poor prognosis (Nature Reviews Cancer, 2007, 7, 763-777; Anticancer Res. 2007, 27, 27-34; Cancer Res., 2006, 66, 5977-5980, Nutrition, 2000, 16, 202-208).
In addition to tumor cells, immune cells metabolically adapt, proliferate and differentiate into distinct functional classes in response to immunogenic stimuli. Studies have demonstrated that lipogenesis plays a critical role in immune responses and metabolic adaptation of activated immune cells. Inhibition of fatty acid synthesis during T-cell differentiation result in a switch from Th17 to Treg cells, suggesting a novel approach to treat autoimmune diseases, such as multiple sclerosis, and to modulate immune responses (Nature Medicine, 2014, 20, 1327-1333). Similarly, de novo fatty acid synthesis is critical for CD8+T cell expansion and dendritic cell activation (Nature Immunology, 2014, 15, 323-332). These results demonstrate that modulation of the fatty acid synthesis pathway might represent a strategy to control immune responses and to treat a wide range of autoimmune diseases.
FASN has been implicated as an important enzyme promoting a life cycle of multiple viruses and microorganisms. De novo lipid biosynthesis has been shown to be necessary for replication of the Flaviviridae family including Hepatitis C Virus, Dengue virus, yellow fever virus, West Nile virus and others (Chemistry and Biology, 2013, 570-582). Inhibition of FASN by small molecule inhibitors such as Cerulenin and Orlistat resulted in a strong inhibition of viral replication. Other viruses also depend on FASN activity including human cytomegalo virus (HCMV) influenza A, Epstein-Barr virus (EBV) and coxsackievirus B3 (CVB3). Numerous genome wide screens identified multiple host genes involved in lipid metabolism which are crucial for replication of viruses and increased expression FASN is often required for efficient viral replication (Nature Biotechnology, 2008, 26, 179-186). Taken together, these results provide a strong rationale for targeting FASN for the antiviral therapy.
Fatty acid accumulation is associated with variety of metabolic diseases and has been shown to contribute to their pathogenesis. The non-alcoholic hepatic steatosis (NASH), also called fatty liver disease, encompasses a spectrum of liver diseases (steatosis, steatosis with inflammation, cirrhosis) characterized by a fatty acid accumulation in hepatocytes. Currently, NASH is the most common liver disease in developed countries and is associated with obesity, insulin resistance and type 2 diabetes. Studies in animal models demonstrated that pharmacological inhibition of FASN improved hepatic function and decreased liver fat accumulation (PloS One, 2013, 9, 1-8).
FASN is highly expressed in tissues with high metabolic activity (liver, adipose tissue and brain), and is a critical enzyme for endogenous lipogenesis and modulation of key intermediates of lipid and carbohydrate cellular metabolism. A FASN inhibitor has been proposed for treatment of obesity, and inhibition of FASN in the hypothalamus may result in reduced food intake. The non-specific irreversible FASN inhibitors cerulenin and C-75 have been reported to decrease brain levels of orexigenic neuropeptides and decrease food intake. Therefore, FASN inhibition represents a therapeutic target in a wide spectrum of pathologies including cancer, antiviral, liver and cardiovascular diseases and treatment of obesity, diabetes and drug-induced body weight gain; e.g. antipsychotics.
Recent advances in the treatment and management of cancer show that many anticancer therapies lead to profound changes in tumor metabolism. Inhibition of BRAF signaling by vemurafenib and inhibition of BCR-ABL by imatinib led to increased oxidative phosphorylation (Pollak M, (2013) Targeting Oxidative Phosphorylation: Why, When and How; Cancer Cell 18, 263-63). Such a drug-induced reprogramming of cellular metabolism from glycolysis to oxidative phosphorylation might create a dependency on lipids which could be exploited therapeutically by use of FASN inhibitors. In yet another example, it was demonstrated that cessation of the anti-angiogenic therapy by sunitinib and sorafenib resulted in a rapid regrowth of tumors and increased metastasis which were mediated by a rapid metabolic switch of tumor and stromal cells to de novo lipogenesis. Pharmacological inhibition of FASN was sufficient to reverse tumor regrowth and metastatic dissemination further confirming the role of lipid metabolism in tumor adaptation to anticancer therapies (Sounni N E, Cimino J, Blacher S, Primac I, Truong A, Mazucchelli G, Paye A, calligaris D, Debois D, mari B, de pauw E, Noel A (2014) Blocking Lipid Synthesis Overcomes Tumor Regrowth and Metastasis after Angiogenic Therapy Withdrawal; Cell Metabolism 20, 1-15) and providing a rationale for combinatorial treatments using FASN inhibitors.